1. Field of the Invention
The disclosed technology generally relates to wireless networks and in particular to beamforming transmissions in wireless networks.
2. Description of the Related Technology
The huge bandwidth available in the 60 GHz band allows short-range wireless communications to deliver data rate beyond 1 Gbps. However, the high path loss and low output power of CMOS power amplifiers (PA) at 60 GHz yield poor link budget, making it impossible to support such high data rate with omnidirectional antenna. A key solution to the link budget problem at 60 GHz is to use multiple antenna beamforming, i.e. to smartly combine (beamform) the signals on the various antennas.
The channel capacity of a wireless link can be improved by using multiple antenna technology. The use of this technology is even more interesting at 60 GHz given the small wavelength of about 5 mm, which enables to pack easily a large antenna array in a very small volume. In general, a multiple antenna array can provide three different types of gain: spatial multiplexing (SM) gain, array gain and diversity gain. On the one hand, SM gain is more attractive for bandwidth limited systems that need a very high spectral efficiency to achieve a given data rate. The SM gain is obtained by transmitting independent streams in the eigenmode subchannels of a multiple input multiple output (MIMO) channel. On the other hand, for large bandwidth systems (e.g. the one operating at 60 GHz), both array and diversity gains are more desirable since they improve the channel capacity by increasing the received SNR.
In most existing MIMO wireless systems (e.g. the one operating at 5 GHz), beamforming (BF) processing is carried out in the digital baseband. The resulting digital beamforming (DBF) architecture implies that each antenna has its own analog front-end (AFE) chain plus a digital-to-analog converter (DAC) at the transmitter or an analog-to-digital converter (ADC) at the receiver. An AFE carries out the translation between digital baseband and analog radio frequency (RF). Several algorithms have been already proposed to compute DBF coefficients at both transmit (TX) and receive (RX) sides. Examples are provided in the papers “Joint Tx-Rx Beamforming Design for Multicarrier MIMO Channels: A Unified Framework for Convex Optimization” (Daniel P. Palomar et al., IEEE Trans. on Signal Proc., vol. 51, pp. 2381-2401, September 2003) and “Joint Beamforming Strategies in OFDM-MIMO Systems” (A. P. Serte, Acoustics, Speech and Signal Processing, in Proc. IEEE ICASSP, vol. 3, pp. 2845-2848, May 2002). Nevertheless, considering that at 60 GHz a wireless system can have an antenna array of more than eight elements, DBF solutions are practically infeasible due to the resulting high cost and power consumption of analog blocks, especially the ADC and the DAC running at several gigasamples per second.
Considering multi-antenna architecture with a lower number of AFE chains than antenna elements, the most used analog spatial processing (ASP) technique is antenna selection (AS), as described e.g. in “Antenna selection in MIMO system”, (Sanayei et al., IEEE Comm. Magazine, vol. 42, pp. 68-73, October 2004). In this technique, only a small number of antennas (which number is equal to the number of available AFE chains) are used to perform DBF and the other ones are simply not used. However, it has been shown that mixed AS/DBF schemes suffer from severe performance degradations in most fading channels, one reason being that they do not provide BF gain. In order to alleviate the problems of conventional AS schemes in fading channels, an analog beamforming (ABF) technique known as antenna subarray formation (ASF) has been recently introduced. This technique exploits the signals on all available antennas by applying a linear transformation in the analog RF domain. However, the paper “Adaptive Antenna Subarray Formation for MIMO Systems” (P. D. Karamalis et al., IEEE Trans. Wireless Comm., vol. 5, no. 11, pp. 2977-2982, 2006) analyzes mixed ABF/DBF techniques at only one side of the transceiver. In “Variable-phase-shift-based RF-baseband codesign for MIMO antenna selection” (X. Zhang et al., IEEE Trans. Signal Processing, vol. 53, pp. 4091-4103, November 2005) a joint TX-RX mixed ABF/DBF algorithm has been proposed for only frequency flat MIMO channels. Nevertheless, due to the large bandwidth used at 60 GHz, the channel is likely to be frequency selective.